In recent years, various types of portable electronic devices, such as a camera-integrated videotape recorder, a cellular phone, and a laptop computer, are widely used, and those having smaller size and weight are being developed. As the portable electronic devices are reduced in size and weight, a battery pack used as a portable power source for the electronic devices is required to have increased energy and reduced size and weight. As an example of the battery used in such a battery pack, there is a lithium-ion secondary battery having a high capacity.
The lithium-ion secondary battery includes a battery element having a positive electrode and a negative electrode capable of being doped with a lithium ion and dedoped. The battery element is sealed in a metallic can or a metal laminate film and is controlled by a circuit board electrically connected to the battery element.
Known lithium-ion secondary batteries have a battery pack including a battery element sealed in a metallic can or a metal laminate film and a circuit board, which are contained in a storage casing having upper and lower spaces divided (see, for example, Japanese Patent No. 3556875, Japanese Patent No. 3614767 and Japanese Patent No. 3643792).
With respect to the battery pack for lithium-ion secondary battery, a battery pack including a battery element sealed in a metallic can and a circuit board which are molded in one piece by using a melted resin has been proposed (see, for example, Japanese Unexamined Patent Application Publication No, 2004-303625 and Japanese Unexamined Patent Application Publication No. 2004-358735).
Further, there has been proposed a battery pack including a battery element sealed in a metal laminate film, a circuit board, and a frame disposed so as to surround the battery element and circuit board, wherein the battery element, circuit board, and frame are covered with a packaging member in the form of a thin plate (see, for example, Japanese Unexamined Patent Application Publication No. 2005-158452).
In addition, there has been proposed a lithium-ion secondary battery having a construction in which a battery element and a circuit board are sealed in a rectangular casing can wherein a cap portion made of an aluminum alloy of the casing can or the casing can body made of aluminum has formed therein a burst-proof ripping open vent or thin-wall portion for releasing gas outside the casing can when the internal pressure rises due to gas generated in the event of an abnormal condition (see, for example, Japanese Patent No. 3691268 and Japanese Patent No. 3783082).
In the known lithium-ion secondary battery described above, the metallic can used for sealing a battery element is formed by deep drawing, which leads a limited elongation in the machine direction for deep drawing, so that only a metallic can having a predetermined thickness (about 200 μm) is formed, disadvantageously lowering the degree of freedom for design. In addition, a metal suitable for deep drawing is required to use, and a metallic can having a strength higher than the strength inherent in the metal constituting the can is difficult to obtain. Further, when the battery element is sealed in a metallic can, it is difficult to perform a heat-pressing step for heat-sealing a gel polymer thereafter, and therefore, only a liquid electrolyte can be used when using a metallic can.
On the other hand, when a battery element is sealed by a metal laminate film, it becomes possible to perform heat-pressing, and there are advantages in that the degree of freedom for size or shape is high. However, the battery element sealed by a metal laminate film has a drawback in that the strength is lower than that of a battery element sealed in a metallic can.
In the battery pack having a lithium-ion secondary battery and a circuit board contained in a storage casing described in Japanese Patent No. 556875, Japanese Patent No. 3614767 and Japanese Patent No. 3643792, for protecting the lithium-ion secondary battery and circuit board from an external impact and the like, a satisfactory thickness is required to the storage casing. Further, in bonding together the upper and lower divided storage casing using an adhesive double-coated tape or by ultrasonic welding, the storage casing requires a satisfactory thickness for facilitating the bonding, and the whole of the battery pack is inevitably increased in thickness or weight. Thus, such a technique is unsuitable for portable power source.
In the battery pack described in Japanese Unexamined Patent Application Publication No. 2004-303625 and Japanese Unexamined Patent Application Publication No. 2004-358735, since a battery element sealed in a metallic can having a large mechanical strength and a circuit board are combined in one unit by using a melted resin, a flexible rubber resin, as the resin, having excellent elastic properties is used for improving the resistance to drop impact. This battery element is totally different from a battery element sealed in a metal laminate film having a small mechanical strength, with respect of dimensional accuracy, production conditions, and the like. Accordingly, such a technique is not suitable to apply to the battery pack using a battery element sealed in a metal laminate film.
Further, in the battery pack using a battery element sealed in a metallic can, an electrolytic solution which is in a liquid state is used as an electrolyte. Therefore, it is necessary to provide a burst-proof structure which does not easily rip open in a usual operation, in the battery pack.
On the other hand, in the battery pack described in Japanese Unexamined Patent Application Publication No. 2005-158452, with respect to the packaging member for covering a battery element sealed in a metal laminate film, a circuit board, and a frame, a packaging member having a two-layer structure which includes a surface protecting layer composed of nylon or the like and a metal layer composed of stainless steel or the like is used. Accordingly, it becomes necessary to have a certain thickness for obtaining the hardness and moldability required. Further, an adhesive or an adhesive tape is used and therefore this battery pack is inevitably increased in thickness, and further improvement for reducing the size and weight is desired.
The use of a metal oxide ceramic in the covering material for battery pack is considered. Since the metal oxide ceramic has more excellent hardness than that of a metal, but it has brittleness and it is necessary to process into a thin covering material closely in contact with the battery pack.
The lithium-ion secondary battery having a burst-proof ripping open vent formed in the rectangular casing can described in Japanese Patent No. 3691268 and Japanese Patent No. 3783082 has not yet been put into practical use. The reason for this resides in, for example, that a large vent valve structure is formed in the direction of the longest side of the rectangle and hence a required capacity is difficult to secure, and, as compared to a battery of a cylindrical shape, a battery of a rectangular structure requires higher cost for the parts and positioning of the individual parts with high accuracy, thus making the battery production difficult.
Accordingly, it is desirable to provide a battery pack which is advantageous not only in that the battery pack has both high dimensional accuracy and high mechanical strength and can be reduced in size and weight, but also in that it provides a high degree of safety even in an abnormal condition, and a method for producing the same.